helper/command: pass command prefix to command registration

helper/command: pass command prefix to command registration

Replace the "struct command *parent" parameter with a string that
contains the command prefix.
This abstracts the openocd code from the knowledge of the tree of
struct command.
This also makes unused the function command_find_in_context(), so
remove it.

Change-Id: I598d60719cfdc1811ee6f6edfff8a116f82c7ed6
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5668
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>